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For birds, flocks promise safety – especially if you’re faster than your neighbor
Joan Strassmann, Washington University in St. Louis
As I walked along Bolivar Flats, just across from Galveston Island in Texas, I watched flocks of sanderlings forage along the frothy wavefront as it surged and retreated. Nearby, Caspian terns, American avocets, and black skimmers rested on the beach, each in its own group.
The birds rose simultaneously as I drew near, and then settled farther down the beach, clearly fearing me.
As an evolutionary biologist and author of the new book “The Social Lives of Birds,” I’m fascinated by how social behavior has evolved in birds. Why is it ever worth being with others that not only compete for food but may pass on diseases or even mate with your partner?
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Safety in numbers
The late Oxford University biologist William D. Hamilton discussed the advantages of flocking with his landmark 1971 paper “Geometry for the Selfish Herd.” He theorized that individuals in a flock stay because each benefits from the shelter of the group. At the time, a prevailing belief was that animals moved in groups for the benefit of the group, not the individual.
Groups provide some safety because they’re harder to attack, they’re more likely to provide warnings of approaching danger, and they have an ability to respond together if threatened.
But everyone in the group does not necessarily benefit equally.
Ed Schipul via Wikimedia Commons, CC BY-SA
The best position is one that puts another bird between oneself and a predator, making your neighbor the more likely target. This keeps birds in a group close together as each tries for living shelter. It is a kind of movement you’ll also see in schools of fish or great herds of ungulates in Africa, like wildebeests.
The peril of being the lone bird
Shorebirds, similar to those I saw in Texas, might be the easiest to study, particularly where the predator can come from a forest that borders the shore.
One of the best-studied flocking shorebirds is the common redshank, Tringa tetanus, often seen feeding on mudflats and saltmarshes in Britain. Redshanks are sandpipers very closely related to the greater and lesser yellowlegs I see in Texas, but with red legs rather than yellow.
Mike Pennington via Wikimedia, CC BY
The predator that redshanks have most reason to fear is the Eurasian sparrowhawk, which watches foraging redshanks from the trees bordering the saltmarsh. When a sparrowhawk picks its prey, it flies fast and hard toward a single predetermined target, grabbing the hapless redshank with its talons.
Evolutionary biologist Will Cresswell studied the redshank’s flocking behavior on the chilly Tyninghame Estuary and found that sparrowhawks were most successful in catching lone birds and those in smaller flocks.
Janne Passi via Wikimedia, CC BY-SA
The closer a bird was to a neighbor, the less likely it was to be targeted and caught. That reminds me of that old trope about how fast you have to run from a lion: just faster than your neighbor.
Large flocks have downsides, too
One downside to being in a large redshank flock is that these birds have to take more steps to get food because they have more competition.
With other flock members probing the sand, and the sand shrimp and other invertebrates fleeing this probing, the redshanks spend more time foraging when they are in larger flocks.
ImagePerson via Wikimedia Commons, CC BY
Canadian ornithologist Guy Beauchamp compared closely related species on islands where there were fewer predators with those on the mainland where predators were common. Flocks were smaller on islands, allowing the birds to forage with less competition.
Fantastic flying flocks
Flying in flocks can also help birds avoid predators.
Evolutionary biologist Daniel Sankey and his colleagues separated the behavior of predator and prey by using an artificial predator, the ingeniously engineered flying robot falcon. Its behavior could be mechanically controlled as it approached a flock of homing pigeons, all labeled with GPS tags that allowed precise measurements of how the birds’ positions changed.
The team compared pigeon flight with and without attacks by the robot falcon and found that when the pigeons noticed the robot falcon, they turned sharply away from it, following the direction their nearest neighbor was turning and did not cluster more tightly.
More spectacular but harder to study are the mesmerizing flocks of European starlings as they circle and swerve, avoiding predators before settling for the night. These flocks of thousands are called murmurations and are fantastic to watch, and likely frustrating for predators that would struggle to grab a single bird from the swirling scene.
Italian physicist Michele Ballerini figured out that this magnificent visual concert was the result of birds simply keeping track of six nearest neighbors, turning and moving when they did.
Beyond flocks: Roosts and supersociality
Birds are social in other ways, too.
Some sleep together in roosts, nest near each other in colonies, or show off together, carrying out mating dances in what is known as lekking to attract females. They may actively help each other in rearing the young, typically if they are related to the breeders, or anticipate inheriting the breeding position or territory.
Take time to watch the behavior of the birds around you, and you’ll start to notice social behaviors everywhere, from the ducks in a city pond to the chickadees hunting for insects deep in winter. I hope you’ll watch them with more understanding of their social lives, and with a little bit more wonder.
Joan Strassmann, Professor of Biology, Washington University in St. Louis
This article is republished from The Conversation under a Creative Commons license. Read the original article.
